Shutter timing apparatus



y 1966 J. P. 'BURGARELLA 3,249,034

SHUTTER TIMING APPARATUS Filed March 29, 1963 3 Sheets-Sheet 1 FIGZINVENTOR.

ATTORNEYS y 1966 J. P. BURGARELLA 3,249,034

SHUTTER TIMING APPARATUS Filed March 29, 1963 3 Sheets-Sheet 2 PORTIONso DEPRESSED s CLOSES MAX. CURRENT IN soLENoID TR|GGER voLTAGE soPENsTIMING STARTS REACHEQ STOPS EXPOSURE INITIATED EXPOSURE TERMINATEDBY oPENING BLADE BY CLOSING BLADE Y Y I u TIM 0.53 WW 11 V E QELAYIACTUAL EXPOSURE TIME a c l TRIGGER GENERATION IME DELAY ggfiggggg"LoPENING BLADE BEGINs To MOVE o, CONDUCTS Q2 coNDucTIoN REDUCED SEES $3MIN CURRENT IN SOLENOID cLosING BLADE BEGINS TO MOVE Q3 coNDucTs 0, cmOFF L I 2 I-z! Ill- :EI 3g 5 TIME F I G 3 .1. HIGH LIGHT LEvEL low LIGHTLEvEL --lOl TRIGGER /j VOLTAGE I 4 I l I l I I I l I L 0 *b QREENERGIZATION CAUSES BLADE RECAPTURE 6 Z I REENERGIZATION TOO LATE TORECAPTURE BLADE 5 E f I l T II I o D I TIME 0) Q L- L... RELEASEINVENTOR.

,dhuufl'am ATTORNEYS ware Filed Mar. 29, 1963, Ser. No. 269,117 6Claims. (Cl. 95-53) This invention relates to shutter control means forcontrolling the exposure time of a camera, and more particularly toshutter control means of the type that is responsive to light from ascene being photographed for causing the exposure time to befunctionally related to the level of scene brightness.

One recently developed shutter control means of the type described isprovided with shutter operator means actuatable to cause the shuttermeans of the camera to initiate exposure, and deactuatable to cause itto terminate exposure, so that the exposure time is essentially the timethat said shutter operator means is actuated. To control this time, thecontrol means is further provided with shutter timing apparatusincluding a voltage sensitive trigger circuit for actuating anddeactuating the shutter operator means; and an R-C circuit having, as acomponent, a photo'conductive element whose resistance is functionallyrelated to the level of brightness of a scene to which the element isexposed. When the R-C circuit is activated substantially at the sametime the trigger circuit actuates the shutter operator means, itgenerates a time variable voltage that reaches a trigger voltage in aperiod of time dependent upon the parameters of the R-C circuitincludingthe resistance of the element as established by the level of scenebrightness.

The voltage sensitive trigger circuit includes a transistor stage, theinput signal of which is the time variable voltage of the R-C circuit.Such voltage causes the stage to be baised to cut-off until the triggervoltage is reached, and when this occurs the stage is forward biasedinto conduction. During the time that the stage is reverse biased,another transistor stage, operatively associated therewith, is permittedto conduct and is effective to actuate the shutter operator means; andwhen the first-mentioned stage conducts, it causes the other stage tostop conducting and to deactuate the shutter operator means, so that theactual exposure time, as established by the actuation time of theshutter operator means, is essentially the time required by the R-Ccircuit, after activation, to generate the trigger voltage. Regenerativefeedback between the stages causes conduction to rapidlyswitch inresponse to the generation of the trigger voltage.

Shutter means, with which the above-described apparatus is particularlywell suited for use, may include a pair of movable blades, each having ablocking position overlying the exposure aperture of the camera and anunblocking position uncovering the aperture. Prior to the initiation ofexposure, the opening blade is in blocking position and releasablyretains the closing blade in unblocking position. When the opening blademoves to un-. blocking position, exposure is initiated because theshutter operator means, operably associated with the closing blade, isactuated and serves to releasably retain the latter in unblockingposition; and exposure is terminated when the shutter operator means isdeactuated to release the closing blade for movement to blockingposition.

With shutter means in the formabove-described, the shutter operatormeans may advantageously take the form of an electromagnet which isenergized to attract the closing blade and maintain the latter inunblocking position after the opening blade moves to unblockingposition, and deenergized to release the closing blade for movement toblocking position. When the electromagnet is to be energized at the lastpossible moment (that is,

nited States Patent 'ice Patented May 3, 1966 just prior to the timewhen the opening blade is no longer effective to retain the closingblade in unblocking position) the exposure time is essentially the timethat the electromagnet is energized, which in this context is intendedto describe the time that the shutter operator means is actuated.

One of the problems associated with cameras utilizing an electromagnetfor the shutter control means is concerned with malfunctions in theoperation thereof characterized by failure of the electroma-gnet to holdthe closing blade against release as the opening blade moves tounblocking position, or to release the closing blade when the R-Ccircuit generates the trigger voltage. The former type of malfunction,in prematurely releasing the closing blade, can result in extremeunderexposure, or even a failure to achieve any exposure. The lattertype of malfunction, in belatedly releasing the closing blade, canresult in overexposure or even a failure to release the closing blade.This failure to release the closing blade occurs generally at low levelsof brightness.

With particular regard to the first-mentioned malfunction, an obviousexpedient available to those skilled in the art to insure againstaccidental release of the closing blade and the premature termination ofexposure, is to energize the electromagnet sooner and with a largercurrent. While this expedient would probably overcome the malfunctionproblem, it would do so at a cost of increased power consumption, aheavy penalty for a battery operated device. invention to provideshutter timing apparatus of the type described, which, without asignificant increase in power consumption, is capable of preventingaccidental release of the closing blade when exposure is to beinitiated.

Briefly, the invention involves the concept that a hysteresis effect canbe utilized to increase the magnetic induction in the core of theelectromagnet without significantly increasing the current drain on thebattery. This concept is embodied in the shutter'control means of thetype described by providing a system of switches con-v structed andarranged so that, in response to the release of the opening blade, ininitial pulse of .current is caused to be injected into the solenoid ofthe electromagnet of an am? plitude'in excess of the normal holdingcurrent as provided by conduction of the normally conducting transistorstage of the trigger circuit. In other words, release of the openingblade causes the current through the solenoid to be pulsed to a higherlevel than the steady state level, so that the retaining force exertedon the closing blade by the electromagnet is larger by virtue of thehysteresis effect. However, the duty cycle of this pulse is so low thatthere is no appreciable drain on the battery.

The more important features of this invention have thus been outlinedrather broadly in order that the detailed description thereof thatfollows may be better understood, and in order that the contribution tothe art maybe better appreciated. There are, of course, additionalfeatures of the invention that will be described hereinafter and whichwill also form the subject of the claims appended hereto. Those skilledin the art will appreciate that the conception upon which thisdisclosure is based may readily be utilized as a basis for designingother structures for carrying out the several purposes of thisinvention. It is important, therefore, that the claims to be grantedherein shall be of suflicient breadth to prevent the appropriation ofthis invention by those skilled in the art.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accom panying drawings wherein:

FIGURE 1 is a schematic representation of a camera at a typeparticularly well adapted for use with the present invention;

It is therefore a primary object-of this FIG. 2 is a schematicelectrical diagram of the present invention;

FIG. 3 is an expanded time-scale diagram for the purpuse of illustratingthe chronology of events associated with efiecting exposure utilizingthe camera of FIGURE 1 and the electrical system of FIG. 2;

FIG. 4 is a voltage-time plot showing the variation of voltage generatedby the R-C circuit after activation thereof;

FIG. 5 is a side view of the electromagnetic means shown in FIGURE 1 forthe purpose of illustrating its operable association with the closingblade when the latter is in blocking position;

FIG. 6 is a hysteresis loop illustrating the manner in which themagnetic induction in the core of electromagnetic means varies withsolenoid current; and

FIG. 7 is an expanded time scale plot of the current through thesolenoid to emphasize the low duty cycle of the input current pulse.

The shutter control means to be described is shown in the drawing asembodied into a camera having a particular type of shutter means, butthe latter is for the purpose of; illustrating the invention in a simpleenvironment, it being understood that other types of shutter meanscould'also be used with the shutter control means disclosed and claimedherein in order to derive the new and improved results attendantthereto.

Referring now to FIGURE 1, camera 10 is shown schematically as includinglens 11 for receiving light from a scene being photographed, andfocusing it through an aperture in diaphragm means 13 onto film 12 toeffect exposure of the latter by the proper operation of shuttermechanism 14 which is interposed in the optical path of the light.

Diaphragm 13 may take the form of a disc 15 mounted on the camerahousing. Disc 15 contains exposure aperture 15' of preselected areaaligned with the optical axis A-A of the camera.

Shutter'mechanism- 14 may include shutter means 17, and shutter controlmeans 18. Shutter means 17 may take the form of a pair of planar, opaqueblades 19 and 20, each provided with an exposure orifice 21, and mountedin tracks (not shown) so as to be normal to the optical axis, andreciprocable between terminal positions into intersection therewith.Each of the blades has one terminal position at which the solid portionoverlies and totally blocks the exposure aperture in diaphragm means 13(closed position), and another terminal position at which the exposureorifice is aligned with the exposure aperture (open position). Betweenthese two terminal positions, each blade has an'intermediate position atwhich it covers only a portion of the exposure aperture. In accordancewith convention, it is assumed that the intermediate position at whichhalf of the exposure aperture is covered is the position at whichexposure is either initiated or terminated as the case may be. The bladethat causes exposure to be initiated is called the opening blade. Theterminal position at which the opening blade is closed is termed theblocking position, while the position intermediate the two terminalpositions of the opening blade at which it initiates exposure is termedthe unblocking position. Conversely, the blade'that causes exposure tobe terminated is called the closing blade. The terminal position atwhich the closing blade is open is termed the unblocking position, whilethe position intermediate the two terminal positions of the closing Iblade at which it terminates exposure is termed the blocking? position.

Prior to initiation of exposure, the blades are as shown in the solidlines of FIGURE 1, inspection of which will indicate that releasablycoupling means 22 is engaged with opening blade 19 tohold the latter inblocking position against the bias of spring means 23 which urges theblade toward unblocking position. Coupling means 22 includes latch 24pivotally mounted on pin 25 and en gaged with latch pin 26 attached toblade 19. Latch spring 27 engaged with latch 24 urges the latter intolatching contact with pin 26. Reset bar 28 is rigidly attached to theend of blade 19 opposite exposure orifice 21, and extends normal theretointo the path of movement of closing blade 20. When the opening blade isheld in blocking position by coupling means 22, bar 28 is effective toengage blade 20 to maintain the latter open position against the bias ofspring means 29 urging the closing blade toward its blocking position.As can be seen from the drawing, bar 28 does not interfere with theindependent movement of opening blade -19 to its open position.

Such movement takes place upon manual depression of end portion 30 oflatch 24, which rotates the latter about pivot 25 out of engagement withpin 26. Upon disengagement of coupling means 22 from opening blade 19,the latter moves out of blocking position toward open position, and bar28 is no longer effective to maintain closing blade 20 in its openposition. However, initial movement of opening blade 19, in response tothe disengagement of coupling means 22 therefrom, is effective to causeshutter control means 18, in a manner to be described later, to retainclosing blade 20 in its open position for a preselected period of timedepending on the level of scene brightness. Since the opening blademoves to unblocking position while the shutter control means releasablyretains the closing blade in open position, exposure is initiated. Inother words, the shutter operator means is so operably associated withthe shutter means, that the latter is caused to initiate exposure inresponse to actuation of the shutter operator means. At the end of saidpreselected period of time, shutter control means 18 causes closingblade 20 to be released thus terminating exposure when the latter ismoved from open to blocking position by the action of bias spring means29.

When exposure is terminated, blades 19 and 20 are in the position shownby the broken lines of FIGURE 1. That is to say, blade 19 is in openposition and blade 2%) is in blocking position, with reset bar 28 againengaged with blade 20. Having completed the exposure cycle, the bladesare returned to their normal positions by reset mechanism 31 whichincludes reset shaft 32 rotatably mounted on the camera housing, resetlever 33 rigidly fixed to one end of shaft 32 and manual reset actuator34 rigidly fixed to the other end of the shaft. Spring means 35 biaseslever 33 to its normal position out of the path of movement of reset bar28. However, the manual rotation of lever 33 against spring 35, achievedby the manual rotation of actuator 34 after exposure is terminated,causes lever 33 to engage bar 28 and move both blade 19 and blade 20back to their normal, preexposure positions wherein the opening blade isin blocking position and the closing blade is in open position, theblades being held there by the action of coupling means 22. Upon releaseof actuator 34, lever 33 returns to its normal position, and themechanism is ready for the next exposure cycle. If desired, the manualrotation of actuator 34 can be coupled to a film indexing mechanism.

Shutter control means 18 includes shutter operator means '36 actuatableto cause shutter means 17 to initiate exposure, and deactuatable aftersaid preselected period of time to cause the shutter means to terminateexposure, as previously described. Specifically, operator means 36 maytake the form of an electromagnet 37 which has solenoid 38 wound aroundone leg of U-shaped pole-piece 39, the free ends of which are coplanarand cooperable with magnetizable keeper 40 mounted on closing blade 29when the latter is in open position. Pole-piece 39 and keeper 40, insuch case, define a magnetic circuit of a particular reluctance suchthat a preselected magnetomotive force (solenoid current) applied to themagnetic circuit is sufficient to establish an attractive force betweenthe pole-piece 39 and the keeper 46 that exceeds the separating forceexerted on the closing blade by the biasing that the closing blade isheld in unblocking position is obtained by rapidly energizing thesolenoid just prior to the release of the opening blade, and effecting arapid release of the closing blade by the electromagnet at the propertime.

The solenoid must be energized before opening blade 19 begins to moveout of unblocking position, because this blade, through bar 28, servesto initially position keeper 49 in engagement with pole-piece 39. Oncethe keeper has separated even slightly from the pole-piece, thereluctance of the magnetic circuit is so high, that the solenoid currentis unable to produce an attractive force solenoid and prevents theinduced voltage from damaging transistor Q The current in the solenoidhowever, rapidly decays to the point where the magnetic induction is soreduced that the force of attraction of the pole-piece on the keeper isequal to the spring force urging separation. At this point, there is aninitial movement of keeper 4% as the closing blade is drawn towardunblocking position by the spring bias. The time between the instantthat the current in Q is rapidly reduced and the instant that release ofthe closing blade is accomplished is very small in terms of the ordinaryexposure interval, being only a fraction of a millisecond. For thisreason, it is considered that the conduction of Q is reduced, andmovement of the closing blade begins at substantially the same time.

For the reasons set forth above, namely rapid switching of the currentinput to the solenoid and low power consumption, shutter timingapparatus 41 takes the form of a transistorized, two-stage, modifiedSchmitt-type trigger circuit 43, responsive to the output voltage fromnetwork 44, for controlling the actuation and deactuation of shutteroperator means 36. The voltage sensitive trigger circuit 43 has anormally not-conducting stage that includes transistor Q preferably of asilicon type, having base,

collector and emitter electrodes 50b, 50c and 5012 re-' spectively.Collector electrode Site of Q is connected to terminal 47 of the shuttertiming apparatus by variable bias resistor 52, and emitter electrode50:: of Q is connected to terminal 48 of the shutter timing apparatus byvariable bias resistor 53. The normally conducting stage of circuit 43includes transistor Q having base, collector and emitter electrodes 54b,54c, and 54e. Collector electrode 54c is connected to terminal 47through solenoid 38 so that the latter is energized when Q conducts.Base electrode 54b of Q is connected to collector electrode Site of Qthrough lead 55, and emitter electrode 542 of Q is connected throughbias resistor 53 to terminal 48. It should be noted that with thisarrangement there is essentially a common emitter resistor, theadjustment to resistor 53 being for the purpose of establishing thevoltage at which it is desired to trigger circuit 43. While the twostages of circuit 43 have been characterized as normally not-conductingand normally-conducting it should be obvious that this characterizationis appli- 6 cable only when a voltage source is applied across terminals47 and 48.

In order to apply voltage source 56-, shown in the form of a battery ofpotential E connected from terminal 48 to terminal 47 through normallyopen switch S1, in such a way as to minimize current drain on thebattery, actuator lever 24 is provided with switch operating arm 57 thatit engageable with one of the contacts of switch S1. When the lever ismanually depressed to disengage coupling means 22 from opening blade 19,the contacts of switch S1 will be closed. The sequence of events thatoccur as a result of the depression of end portion 30-will now beexplained by making reference to the expanded time-scale diagram shownin FIG. 3. The initial depression of portion 30 closes the contacts ofS1 before the rotation imparted to lever 24 effects its disengagementfrom pin 26. Since human reaction time involved in depressing lever 24,namely the time to depress the lever and release it, and the inertialdelay of the lever in returning to its normal position, substantiallyexceeds the longest average exposure apt to be used under normalsnap-shot conditions of scene brightness, the'contacts of switch S1 willbe closed for at least as long as the correct exposure time.

Inspection of Q stage of circuit 43 indicates that base electrode 54b isthe input to this stage, collector electrode 540 is the output, andemitter electrode 54e is common to the input and output. Resistor 52coupled between input electrode 54b and terminal 47 acts as a fixed baseresistor for providing, when S1 is closed, a fixed base current biasthat causes Q to conduct instantaneously with the closing of S1. Thesetting of variable resistor 52 establishes the degree to which Qconducts so that the current through solenoid '38 can be adjusted toprovide the proper magnetomotive force in the magnetic circuit ofelectromagnet 37 for preventing accidental release of closing blade 20when the opening blade moves to unblocking position to initiateexposure. 4 The flow of current through resistors 52 and 53, when Qconducts,'establishes at the collector and emitter electrodes of Q biasvoltages having first values dependent upon the magnitudes of therespective currents and resistance values.

Until the initial displacement of the opening blade out of its blockingposition, conductive block 74 on the opening blade engages the contactsof S2 whereby the latter is closed. Connection 49 is at an initial valueof voltage, namely ground potential, at the instant S1 is closed. Whenthe voltage at connection 49 is at its initial value and the voltages atthe collector and emitter electrodes of Q are at their first values ofbias voltage due to the conduction of Q the collector-base andemitter-base junctions of Q are reverse biased, thus resulting in Qbeing cut off. For this reason, it may be said that the bias of Q isprimarily established by the voltage at connection 49.

Meanwhile, the current through the solenoid builds rapidly to itsmaximum value causing the maximum retaining force to be exerted on theclosing blade shortly after S1 is closed and just prior to the totaldisengagement of lever 24 from pin 26 which releases the opening bladefor movement out of blocking position. The initial move merit of theopening blade causes conductive block 74. to disengage the contacts ofS2 thereby opening the same to apply the voltage source across timingnetwork 44 and activate the same. Network 44 includes capacitor means Cin series with photoconductive element 45, such as a cadmium sulfidephotocell or the like exposed to light from the scene being photographedand having a resist-. ance inversely related to the level of scenebrightness. Network 44 is connected between terminals 47, 48 of theshutter timing apparatus so as to form a conventional integrator circuitwhose input terminal is at 47, and whose output terminal is at 49, theconnection between the capacitornneans and the photoconductive element.Terminal 49 is connected by fixed impedance 60 to base electrode 50b ofQ the latter electrode constituting the input electrode of that stage.

The opening of S2 activates network 44, which is to say that the latteris caused to generate, at connection 49, a voltage having an initialvalue (in this case ground potential) which causes Q to be reversebiased to cut-off, and then changes with time reaching a preselectedvalue, termed the trigger voltage, which forward biases Q1, in a periodof time termed the trigger generation time. The charge on capacitormeans C changes after S2 is opened in response to the flow of atransient current, a portion of which flows through Q while the latteris reverse biased and constitutes a leakage current, and a portion ofwihch flows through element 45.

For a given transistor, the amount of leakage current is dependent uponthe degree to which the junctions of the stage are reverse biased. Asthe transient current builds a charge on capacitor means C, the reversebiases are steadily reduced. However, the percentage of the currentflowing through Q as a leakage current that is effective in chargingcapacitor means C becomes substantial (exceeding at low levels of scenebrightness, since the resistance of element 45 as established by thelevel of scene brightness, and the changing voltage difference acrossthe element, controls the amount of current that can flow through theelement. It can thus be seen that the trigger generation time will bedependent upon the parallelcombination of the impedance of the stage tothe flow of leakage current and the resistance of the element. As aresult, the time required to reach the trigger voltage is shorter thanwould be the case were all of the charging current to flow through onlyelement 45. In general, at low levels of scene brightness the triggergeneration time is less than the correct exposure time because of theinherent nonlinear nature of the photoconductive element, withoutconsidering the effect of leakage current through Q When the latter isconsidered, the trigger generation is still further shortened andsubstantial underexposure results.

To minimize the effect of leakage current on the trigger generation timeat low levels of scene brightness, Q is selected on the basis of itsleakage current at reverse bias conditions imposed by the other elementsof the shutter timing apparatus. Thus the transistor Q is a type havinga leakage current associated therewith which is substantially smaller(less than 10%) than the current which flows through element 45 when thelevel of scene brightness is at the desired minimum level of brightnessfor which the camera is designed. Thus, at levels of brightness greaterthan such minimum level, substantially all of the transient currentserving to charge capacitor means C flows through the photoconductiveelement, and the trigger generation time is essentially independent ofthe flow of leakage current. An example of a transistor suitable forthis purpose is a silicon-planar transistor, similar to a 2N930.

When the voltage at connection 49 reaches the trigger voltage, it causesthe emitter-base junction of Q to be forward biased. Now, element 45functions like a base resistor whose value is dependent upon the levelof scene brightness and provides base current bias that causes Q toconduct producing collector current at the output electrode thereofwhich flows through resistor 52 increasing the voltage drop thereacrossand lowering the voltage at the input electrode of Q This reduces theforward bias on Q thus decreasing the flow of current through the latterand causing a reduction in the voltage drop across bias resistor 53thereby increasing the forward bias on Q even more. This regenerativefeedback between the stages of voltage sensitive trigger circuit 43 willcause conduction to switch rapidly from Q to Q if sufficient collectorcurrent is available in Q Assuming this is true, the different flows ofcurrent through bias resistors 52 and 53 after switching takes placeestablish second values of bias voltages at electrodes 50c and 50abrightness is reduced to 0.195 candle/ft? & of Q such that theconduction of Q is severely and rapidly reduced thereby rapidlydeenergizing solenoid 38 to effect a sharp release of the closing blade.

When the closing blade is released, it begins to move out oftin-blocking position, and will terminate exposure at its blockingposition as explained previously. Thus, the shutter means terminatesexposure in response to deacutation of the shutter operator means.Referring again to FIG. 3, it can be seen that the time interval betweenwhen timing starts (the opening of S2) and when timing stops (thetrigger voltage is reached), will be exactly the same as the timeinterval between when exposure is initiated and when exposure isterminated, provided only that the opening blade delay is the same asthe closing blade dela even though the two time intervals are notconcurrent throughout. This disclosure assumes the delay times areessentially the same, so that the actual exposure time is essentiallythe same as the trigger generation time.

Referring again to FIG. 3, it is apparent that the time interval betweenthe release of the opening blade and its initial movement to open S2 canbe made extremely small by proper bias applied to the blade.Furthermore, the time between the closing of S1 and the attainment ofmaximum restraining force on the closing blade can likewise be madeextremely small by various expedients. Thus, all the events that occurbetween the closing of S1 and the opening of S2 can be said to occursubstantially at the same time, particularly since the time intervalinvolved is a minute fraction of the actual exposure time. Thisdisclosure therefore assumes that the actual exposure time isessentially the time that the current through the solenoid is sufficientto maintain the closing blade in unblocking position; or in other words,the time that the shutter operator means is actuated.

As indicated previously, the output current of Q when the latter isforward biased into conduction mustexceed a predetermined value in orderto reduce the conduction of Q to the point where spring means 29 becomeseffective to separate keeper 40 from pole-piece 39 of the electromagnet.In other words, it is quite possible for the voltage at connection 49 toreach the trigger voltage and forward bias Q without causing release ofthe closing blade. This can occur when the resistance of element 45 isso high (low levels of scene brightness) that the current that is ableto flow therethrough to establish fixed base current bias conditions forthe Q stage cannot cause the stage to produce an output current thatexceeds the necessary predetermined value. Stated otherwise, the gain ofthe Q stage must be sufficient so that when the level of scenebrightness is at the minimal design level and the current through theelement is at its lowest value, the output current must exceed thepredetermined value identified above.

An unexpected advantage in this arrangement is obtained by the use of asilicon planar transistor for Q since the current gain of transistors ofthis type is high enough to achieve the desired results, even when thelevel of scene Thus the trigger generation time is made substantiallyindependent of the leakage current that results from reverse biasing Qand positive release of the closing blade is assured when the electricalnetwork generates the trigger voltage.

It should be understood that Q need conduct only to the degree.necessary to effect a reduction in the conduction of Q suficient for themagnetic induction of the electromagnet to be reduced to the point wherethe spring bias on the closing blade urging the latter toward blockingposition overcomes the magnetic force exerted on the closing blade bythe magnet. In other words, Q need not be biased to cut-off to effectblade release. Thus, there exists a reduced level of conduction for Q atwhich spring 29 is strong enough to overcome the magnetic force actingon the closing blade. The resulting current flowing through solenoid 38can be said to deenergize the solenoid (deactuate the shutter operatormeans). Regardless of the amount of current flowing through solenoid 38on deactuation, blade recapture. can occur if conduction at the reducedlevel .fails to continue for a given period of time depending on theresponse time of the blade to conduction at such reduced level. Thismeans that any action that results in Q conducting at a level greaterthan the level-at which the solenoid is deenergized within said giventime period will cause the force exerted on the closing blade by theelectromagnet to increase to the point where spring 29 is ineffective insecuring the release of the closing blade. This can occur even thoughthe trigger volt-age has been initially reached. In practice, thisdifiiculty occurs frequently when the level of scene brightness is low,which is to say, when the output current of Q is just slightly less thanthat necessary to cause Q to conduct at the reduced level necessary todeactuate the shutter operator means.

In attempting to obviate this difficulty, it was discovered that when Qis forward biased by the trigger voltage and begins to conduct, theregenerative feedback connection between Q and Q while providing a rapidswitch in conduction between the stages, also permits capacitor means Cof network 44 to discharge into the base electrode 501: of Q intoemitter electrode 50:: and to ground through resistor 53. Because theinput impedance of Q is generally low for a transistor type havingsatisfactory current gain and resistance to leakage currentcharacteristics, the

' discharge of C is very rapid. The collector current of Q is theproduct of its gain and the base current, the latter being composed ofthe current from the discharge of C and the current flowing throughelement 45 as a result of the volt-age difference thereacross. Suchcollector current will be suflicient, even at very low light levelswhere the contribution of the current flowing through element 45 is verysmall, to rapidly drive Q to cut-oif, or at least to such a reduceddegree of conduction that for practical purposes, it may be said thatthe solenoid is deenergized (shutter-operator means deactuated).However, after C is discharged, the base current in Q is limited to thecur- 'rent able to flow through element 45. Thus, below a certain lightlevel, the resistance of element 45 will be so high that Q does not havesufficient gain to produce the collector current necessary to keep Q inits cut-off condi-.

tion; and after being cut-off for a very short time (discharge time ofC) Q conducts and reenergizes the solenoid before keeper 40 hasseparated from the pole piece of the electromagnet, and blade recaptureresults.

By coupling connection 49 to the input electrode of Q through impedance60, the rate at which capacitor means C can discharge will be limited.The value of this impedan-ce is'made large in comparison to the inputimpedance of Q which latter is forward biased so that impedance 60constitutes means effective at low levels of scene brightness forcausing trigger circuit 43 to deactuate shutter operator means 36 for atleast the required time.

The use of impedance 60 has no effect on the operation of the deviceunder conditions where the resistance of element 45 is low enough sothat the current therethrough times the gain of Q is sufiicient tomaintain Q cut-off. The presence of the impedance increases the releasetime of the keeper from the pole piece, which is to say the time betweenwhen the current in the solenoid is switched oil to when the keeper hasmoved sufiiciently far to prevent blade recapture. While this factormight appear to detrimentally affect exposure by lengthening the timeinterval during which light is permitted to reach the film, this is notthe case, since this situation arises only at low light levels where thecorrect exposure time is extremely large in comparison to theincremental increase in release time. For comparison purposes, the lightlevel at which blade hang-up was a problem might require an exposure ofthe order of magnitude of ,6 second while the incremental increase inrelease time might be of the order of magnitude of second.

. voltage at connection 49 as a function of time varies generally inaccordance with curve 160. The discontinuity occurring at time t is dueto the change in the input impedance of Q that occurs when the triggervoltage is reached. The nature of the discontinuity is exaggerated inthe drawing for the purposes of illustrating what occurs, although thediscontinuity practically disappears at very high levels of scenebrightness. The contribution of the current in resistor 53 due to thedischarge of capacitor means C decreases as the resistance of elernentdecreases since the degree to which Q conducts is directly related tothe resistance of the element. Thus, the discharge of the capacitormeans at high light levels has little effect on changing the currentoutput of Q However, at low light levels, the resistance of element 45is high, and the current through resistor 53 due to the discharge of thecapacitor mean-s has the efi'ect shown at curve 101. Without impedance60, the voltage at connection 49 drops rapidly at the base of Q as soonas the trigger voltage is reaced and Q is forward biased. This isindicated in an exaggerated manner at 102. With impedance 60 present,the voltage drops less rapidly as indicated at 108. As a result, theelectromagnet is deenergized for at least the time required for theclosing blade to move slightly and separate keeper 40 from .the polepiece 39.

To prevent accidental release of the closing blade when the openingblade moves to initiate exposure, it is essential that solenoid 38 ofelectromagnet 37 be energized immediately prior to initial movement ofthe opening blade. In this context,- the solenoid is considered to beenergized when there is sufficient current passing therethrough toestablish a magnetic force large enough to attract keeper 40 when theclosing blade is in its unblocking position, and to be deenergized wherethe current in the solenoid is so reduced that spring 29 exerts agreater force on the closing blade than the force due to the solenoidcurrent. The term immediately prior ascribed to the time when thesolenoid is energized means that the solenoid is energized in the timeinterval between manual depression of portion 30 (the event that occursat the time the operator of the camera decides to eifect exposure), andthe initial movement of the opening blade out of blocking position. Y

With a voltage sensitive trigger circuit like that shown in thisdisclosure, currentis supplied from the battery to the two stages onlyduring the time that switch S1 is closed. As should be apparent fromabove, it is possible for Q to conduct during a large portion of thetime that S1 is closed, deenergization of the solenoid taking place forthe prescribed time to effect blade release. However, in view of thefact that once keeper 40 has separated from the pole piece,reenergization of the solenoid has no effect on the exposure time. Forthis reason, it is accurate to state that the exposure time isessentially the time-that the solenoid is energized. Since this is thetime during which the solenoid is doing useful work, current drain canbe minimized if the current required to energize the solenoid-were keptas low as possible consistent with the requirement for preventingaccidental blade release; This is accomplished by the'technique ofmagnetic pulsing which utilizes the hysteresis effect in the response ofthe magnetic induction to changes in the exciting current. To this end,normally closed switch S5, as shown in FIGURE 1, may be (mounted so thatone contact arm is positioned in the path of movement of latch 24 suchthat movement of the latter to the position at which pin 26 isdisengaged causes S5 to be opened substantially at the same time S2 isopened. S5 is connected between the output elect-rode of Q that iscollector 54c, and terminal 48. Thus S5 is closed only during a veryshort time interval as shown inFIG. 7.

During this time, the full potential of battery 56 is applied directlyacross solenoid 38, and the current is limited only by the internalresistance of the latter. Thereafter, the current decreases to the levelestablished by the setting of resistor 52, since S5 then opens. Theoperation then proceeds as previously described.

Referring now to FIG. 6, the maximum current passing through thesolenoid is i which causes the magnetic induction in the completedmagnetic circuit consisting of 7 core 39 and keeper 40 to be at a valueBm. This current flows only during the time that S5 is closed, such timebeing so short that the current through the solenoid is considered'to bepulsed. Then, S5 is opened (as shown in FIG. 7), causing the current inthe solenoid to decreas to its value as determined by resistor 52, i Asa result, the magnetic induction decreases to B which is considerablyhigher than the value B which would be the induction in the magneticcircuit were the magnetic pulsing technique omitted. Thus by providingmeans for producing a low duty cycle pulse of current whose amplitudeexceeds the value established by the conduction of the transistors, andapplying such pulse to the solenoid substantially coincident withconnection of the power source (voltage sensitive trigger circuit) tothe solenoid, the force exerted on the keeper is higher. This means thatwith magnetic pulsing i can be lower than the current without suchpulsing to achieve the same force. Obviously, significant improvement inbattery life is achieved.

Since certain changes may be made in the above method and apparatuswithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

l. A shutter mechanism for a camera having an exposure aperture forphotographing a scene, comprising:

(a) at least one shutter blade movable into blocking and unblockingposition relative to said exposure aperture for controlling the passageof incident light therethrough;

(b) electromagnetic means operably associated with said shutter meanswhen the latter is in unblocking position such that energization of saidelectromagnetic means causes the latter to attract said blade andmaintain the same in unblocking position;

(c) a pair of terminals;

((1) a first transistor having an input electrode constituted by thebase thereof, an output electrode, and a common electrode;

(e) means coupling said electromagnetic means between said outputelectrode and one of said terminals so that said electromagnetic meansis energized when said first transistor conducts at a first level anddeenergized when conduction of the latter is reduced to a second level;

(f) means coupling said common electrode to the other terminal;

(g) means coupling a fixed base resistor between said input electrodeand said one terminal for providing,

when a voltage source is connected between said terminals, a fixed basecurrent bias that causes said first transistor to conduct at said firstlevel and energize said electromagnetic means;

(h) a second transistor having an input electrode constituted by thebase thereof, an output electrode and a common electrode;

(i) means coupling the output electrode of said second transistor to theinput electrode of said first transistor so that the output current ofsaid second transistor flows throu h. said fixed base resistor andreduces the forward bias on said first transistor to a level whichreduces conduction of said first transistor;

(j) meanscoupling the common electrodeof saidsec- 0nd transistor to thecommon electrode of said first transistor;

(k) an electrical network connected between said terminals;

(1) said network including capacitor means, a photoresponsive elementexposed to light from said scene and having a resistance inverselyrelated to the level of scene brightness, said element being connectedto said one terminal, and a connection between said capacitor means andsaid element;

(m) means connecting said connection to the input electrode of saidsecond transistor;

(n) said network being so constructed and arranged that when activated,the voltage at said connection has an initial value which reverse biasessaid second transistor to cut-oil? and then changes with time reaching apreselected value, termed the trigger voltage, which forward biases saidsecond transistor in a period of time termed the trigger generationtime;

(0) means to connect a voltage source between said terminals to activatesaid network substantially at the same time said first stage is biasedinto conduction whereby said first transistor conducts at said firstlevel to energize said electromagnetic means for a period of time atleast equal to the trigger generation time and then conducts at saidsecond level to deenergize said electromagnetic means;

(p) pulsing means for causing a relatively short duration pulse ofcurrent whose amplitude exceeds said first level to be appliedimmediately prior to the conduction of said first transistor; and

(q) means effective to move said shutter means to blocking positionafter said electromagnetic means is deenergized.

2. Apparatus in accordance with claim 1 wherein said pulsing meansincludes said voltage source and switch means by which said source isselectively connectable across said electromagnetic means immediatelyprior to conduction of said first transistor.

3. A shutter mechanism for a camera having an exposure aperture,comprising:

(a) means for unblocking said exposure aperture;

(b) electromagnetic means including a core and a coil wound uponsaidcore;

(c) a shutter blade adapted to be magnetically retained in a lightunblocking position by a holding current of a predetermined levelthrough said coil of said electromagnetic means;

(d) biasing means effective to bias said shutter blade toward a lightblockingposition; and

(e) means for applying a current pulse to said coil of a magnitudegreater than said predetermined level, said pulse causing hysteresisefiects in the magnetic circuit through said coil which maintain themagnetic induction in the magnetic circuit at an elevated level for atime after the application of said pulse, whereby said electromagneticmeans produces an augmented retentive force on said blade continuingafter the application'of said pulse to prevent the accidentaldislodgment of said blade from said electromagnetic means.

4. A shutter mechanism for a camera having an exposure aperture,comprising: 0

(f) biasing means effective to bias said blade toward a light blockingposition; and

(g) switching means operable to momentarily connect said power sourcedirectly across said coil, said power source supplying a pulse ofcurrent to said coil of a magnitude greater than said predeterminedlevel, said pulse causing hysteresis effects in the magnetic circuitthrough said coil which maintain the magnetic induction in the magneticcircuit at an elevated level for a time after the application of saidpulse, whereby said electromagnet produces an augmented retentive forceon said blade continuing after the application of said pulse to preventthe accidental dislodgment of said blade from said electromagnet.

l4 retained in a light unblocking position by a holding current of apredetermined level through said coil of said electromagnet;

(e) biasing means efiFective to bias said second shutter means toward alight blocking position;

(f) means for applying a current pulse to said coil of a magnitudegreater than said predetermined level,

said pulse'causing hysteresis effects in the magnetic circuit throughsaid coil which maintain the magnetic induction in the magnetic circuitat an elevated level for a time after the application of said pulse,whereby said electromagnetic means produces an augmented retentive forceon said blade continuing after the application of said pulse to preventthe accidental 5. The invention defined by .claim 4 wherein said switch-15 dislodgment of Said blade from' Said electromagnet; ing meanscomprises a first, normally open, switch in series and with'said coil, asecond, normally closed switch in series (a) trigger means fortie-energizing said coil to release with said coil, and a switchactuator, said actuator being Said Second Shutter means and terminateexposureoperable to successively close said first switch and then opensaid second switch. 2O

6. A shutter mechanism for a camera having an ex- References Cited bythe Examiner UNITED STATES PATENTS Posure aperture, Comprising!2,055,039 9 93 Roms 5 (a) fi Shutter means; 2,572,229 10/1951 Willcox9556 (b) means for moving said first shutter means from a 2,685,239,3/1954 Doyle 95 53 light blocking position to a light unblocking posi-25 2,356,331 10/1958 Gipeto initwte exposure; 2,999,445 9/1961Fahlenberg 95-63 (0) an electromagnet including a core and a coil wound3,105,286, 10/1963 Bunker 5 upon said core; (d) second shutter meansadapted to be magnetically 30 JOHN M. HORAN, Primary Examiner.

1. A SHUTTER MECHANISM FOR A CAMERA HAVING AN EXPOSURE APERTURE FORPHOTOGRAPHING A SCENE, COMPRISING: (A) AT LEAST ONE SHUTTER BLADEMOVABLE INTO BLOCKING AND UNBLOCKING POSITION RELATIBE TO SAID EXPOSUREAPERTURE FOR CONTROLLING THE PASSAGE OF INCIDENT LIGHT THERETHROUGH; (B)ELECTROMAGNETIC MEANS OPERABLY ASSOCIATED WITH SAID SHUTTER MEAN WHENTHE LATTER IS IN UBLOCKING POSITION SUCH THAT ENERGIZATION OF SAIDELECTROMAGNETIC MEANS CAUSES THE LATTER TO ATTRACT SAID BLADE ANDMAINTAIN THE SAME IN UNBLOCKING POSITION; (C) A PAIR OF TERMINALS; (D) AFIRST TRANSISTOR HAVING AN INPUT ELECTRODE CONSTITUTED BY THE BASETHEREOF, AN OUTPUT ELECTRODE, AND A COMMON ELECTRODE; (E) MEANS COUPLINGSAID ELECTROMAGNETIC MEANS BETWEEN SAID OUTPUT ELECTRODE AND ONE OF SAIDTERMINALS SO THAT SAID ELECTROMAGNETIC MEANS IS ENERGIZED WHEN SAIDFIRST TRANSISTOR CONDUCTS AT A FIRST LEVEL AND DEENERGIZED WHENCONDUCTION OF THE LATTER IS REDUCED TO A SECOND LEVEL; (F) MEANSCOUPLING SAID COMMON ELECTRODE TO THE OTHER TERMINAL; (G) MEANS COUPLINGA FIXED BASE RESISTOR BETWEEN SAID INPUT ELECTRODE AND SAID ONE TERMINALFOR PROVIDING, WHEN A VOLTAGE SOURCE IS CONNECTED BETWEEN SAIDTERMINALS, A FIXED BASE CURRENT BIAS THAT CAUSES SAID FIRST TRANSISTORTO CONDUCT AT SAID FIRST LEVEL AND ENERGIZE SAID ELECTROMAGNETIC MEANS;(H) A SECOND TRANSISTOR HAVING AN INPUT ELECTRODE CONSTITUTED BY THEBASE THEREOF, AN OUTPUT ELECTRODE AND A COMMON ELECTRODE; (I) MEANSCOUPLING THE OUTPUT ELECTRODE OF SAID SECOND TRANSISTOR TO THE INPUTELECTRODE OF SAID FIRST TRANSISTOR SO THAT THE OUTPUT CURRENT OF SAIDSECOND TRANSISTOR FLOWS THROUGH SAID FIXED BASE RESISTOR AND REDUCES THEFORWARD BIAS ON SAID FIRST TRANSISTOR TO A LEVEL WHICH REDUCESCONDUCTION OF SAID FIRST TRANSISTOR; (J) MEANS COUPLING THE COMMONELECTRODE TO SAID SECOND TRANSISTOR TO THE COMMON ELECTRODE OF SAIDFIRST TRANSISTOR; (K) AN ELECTRICAL NETWORK CONNECTED BETWEEN SAIDTERMINALS; (I) SAID NETWORK INCLUDING CAPACITOR MEANS, A PHOTORESPONSIVEELEMENT EXPOSED TO LIGHT FROM SAID SCENE AND HAVING A RESISTANCEINVERSELY RELATED TO THE LEVEL OF SCENE BRIGHTNESS, AND ELEMENT BEINGCONNECTED TO SAID ONE TERMINAL, AND A CONNECTION BETWEEN SAID CAPACITORMEANS AND SAID ELEMENT; (M) MEANS CONNECTING SAID CONNECTION TO THEINPUT ELECTRODE OF SAID SECOND TRANSISTOR; (N) SAID NETWORK BEING SOCONSTRUCTED AND ARRANGED THAT WHEN ACTIVATED, THE VOLTAGE AT SAIDCONNECTION HAS AN INITIAL VALUE WHICH REVERSE BIASES SAID SECONDTRANSISTOR TO CUT-OFF AND THEN CHANGES WITH TIME REACHING A PRESELECTEDVALUE, TERMED THE TRIGGER VOLTAGE, WHICH FORWARD BIASES SAID SECONDTRANSISTOR IN A PERIOD OF TIME TERMED THE TRIGGER GENERATION TIME; (O)MEANS TO CONNECT A VOLTAGE SOURCE BETWEEN SAID TERMINALS TO ACTIVATESAID NETWORK SUBSTANTIALLY AT THE SAMD TIME SAID FIRST STAGE IS BIASEDINTO CONDUCTION WHEREBY SAID FIRST TRANSISTOR CONDUCTS AT SAID FIRSTLEVEL TO ENERGIZE SAID ELECTROMAGNETIC MEANS FOR A PERIOD OF TIME ATLEAST EQUAL TO THE TRIGGER GENERATION TIME AND THEN CONDUCTS AT SAIDSECOND LEVEL TO DEENERGIZE SAID ELECTROMAGNETIC MEANS; (P) PULSING MEANSFOR CAUSING A RELATIBELY SHORT DURATION PULSE OF CURRENT WHOSE AMPLITUDEEXCEEDS SAID FIRST LEVEL TO BE APPLIED IMMEDIATELY PRIOR TO THECONDUCTION OF SAID FIRST TRANSISTOR;AND (Q) MEANS EFFECTIVE TO MOVE SAIDSHUTTER MEANS TO BLOCKING POSITION AFTER SAID ELECTROMAGNETIC MEANS ISDEENERGIZED.